This invention relates to the field of computing by means of artificial neural networks and useful in such applications as signal processing, filtering and image and speech pattern recognition. More particularly, the invention relates to an analog neural network composed of an array of capacitors storing weighted voltage values and provided with an improved dynamic refresh capability for the weighted electric charges on the capacitors.
Artificial neural networks (nets) have been studied for many years in an attempt to achieve performance levels approaching that of humans, especially in such fields as speech and image recognition. These networks are composed of many non-linear computational elements operating in parallel and arranged in patterns that attempt to simulate biological neural networks. These computational elements are connected via weights that are usually adapted or modified during operation to improve performance. Further background information related to the field of artificial neural nets can be found in the article by Richard P. Lippmann, "An Introduction to Computing With Neural Nets", in the IEEE ASSP Magazine, April, 1987.
Neural networks are capable of exploring many competing hypotheses simultaneously using voluminous parallel nets composed of many computational elements connected by links with variable weights. The computational elements (nodes) are generally non-linear and are typically analog devices. The simplest node sums N weighted inputs and passes the result on through a non-linear device. Algorithms used with neural networks may adapt connection weights in time so as to improve system performance based on present results. Adaptation or learning ability is an important feature of these networks.
A serious problem with a neural net composed of an array of capacitors with individual weighted charges stored thereon is that the charges leak off the capacitors and thereby create erroneous weighting factors. In order to overcome this drawback, it has been proposed to operate the apparatus in an enclosed super cooled environment (e.g. in a Dewar filled with liquid nitrogen) so as to minimize the leakage of charge from the capacitors of the neural network. Another suggestion was to use very large capacitors so as to maintain the capacitor charge or voltage for longer periods of time. The disadvantages of these solutions are apparent.